6

The Leningrad contains a shift register1, which I believe is loaded with data from the bitmap. This part loads 8 bits in parallel, and shifts each bit out in turn on the rising edge of a clock signal. This clock signal is called /ТИ (meaning Тактовый Импульс, or clock pulse, spelled TI in our alphabet) and is 7MHz (divided from 14 via a D flip flop).

Now, its output, a bitstream which makes up the picture, is fed to another shift register2. This one is a four-bit shift register, but only one bit is used. Its output is called INFV, which is fed to the demultiplexer that selects the attribute color. It's clocked to ТИ (the inverse of the signal which clocks the first shift register).

Why are two shift registers needed here? The four-bit one in particular makes no sense to me.

On the following schematic, this arrangement may be found on the far right, about half-way down. It's parts DD33 and DD35. enter image description here

Edit: I have also seen variants using two ИР16s I place of the ИР9, three in total, possibly a BOM cost optimisation.


1: КР1533ИР9, or 74LS165, a parallel load 8-bit shift register
2: КР1533ИР16 or 74LS295, a 4-bit right-shift left-shift register with 3-state outputs

  • 1
    "ТИ" - "Тактовый Импульс". (I know you can read Russian). – tum_ May 8 at 8:32
  • Only a guess, but is an attribute shared between groups of consecutive pixels, as in the ZX Spectrum? You might assume that the monochrome bitmap data is fetched a byte at a time and fed out a bit at a time, but the attribute data is loaded for a character cell? – Mark Williams May 8 at 8:34
  • 2
    @MarkWilliams The Leningrad was a Soviet ZX Spectrum clone so your assumption might be totally spot on – DmytroL May 8 at 9:56
10

The second shift register is wired to provide a 4-cycle delay after the pixel data is loaded before it is displayed. This is presumably designed to allow the system enough time to load the corresponding attribute data into the pair of storage multiplexers labelled D30 and D31 after the pixels are fetched but before they start displaying; if it were missing, I suspect the first 4 pixels in each block would show using the attributes from the previous block.

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